Many techniques are available in industry for the separation of the components of a mixture of particulate solids. For example, where the materials to be separated differ substantially in particle size, separation may be achieved using screens or sieves. In cases where the components of the mixture differ in density, it may be possible to achieve separation using a fluidized bed or by means of froth-flotation. Electrostatic separators are also known, which use high voltage fields to attract or repel particles in order to effect separation of materials whose particles differ substantially in the electric charges acquired through various electrification processes.
British Patent Specification No. 2,099,729A and the corresponding U.S. Pat. No. 4,357,234, (the teaching of which documents is incorporated herein by reference) describe an electrostatic method and an apparatus that can be used to separate particles that have different physical properties, for example conductivity, mass, size or density.
The said method comprises the steps of charging the particles; and driving the particles in a forward direction through an alternating electric field--in particular a field of non-uniform intensity in a direction perpendicular to the forward direction--having field lines curved in the perpendicular direction whereby the particles are subjected to a centrifugal force in the perpendicular direction, the centrifugal force on each particle being dependent on the mass, size and electric charge of the particle whereby different particles are separated along the perpendicular direction.
The said apparatus comprises means for generating an alternating electric field having a predetermined length and width, wherein the field lines are curved in the direction of the width of the field; means for inserting the particles into one end of the electric field at the side away from the curvature of the field lines; and means for driving the particles through the electric field along the length of the electric field.
In a preferred form, that apparatus comprises a first electrode in the form of a metallic plate mounted on a conventional vibratory feeder.
A second electrode, also in the form of a metallic plate, is mounted above the first electrode at an acute angle (typically 12.degree.) thereto in a lateral direction. In operation, the electrodes are connected to a high voltage AC source which produces an alternating electric field between the electrodes. The field lines are curved, owing to the inclination of the second electrode with respect to the first.
A chute is arranged to deliver a mixture of particulate materials on to the upper surface of the first electrode at one end thereof and adjacent the side where there is the least separation between the first and second electrodes. The vibratory feeder is so arranged as to transport particles along the length of the first electrode.
The particles moving along the length of the first electrode will acquire charges owing to triboelectrification and/or conductive induction. The curved field lines impart a circular motion to the charged particles which has the effect of subjecting those particles to a centrifugal force. Thus the particles will tend to move in a lateral direction, specifically in the direction in which the two electrodes diverge.
The higher the charge on a particle (compared with otherwise similar particles), or, for equal charges, the smaller or less dense the particle is, the greater will be the motion in the said lateral direction. For example, if pulverised fly ash (PFA) contaminated with carbon is fed to the apparatus, the heavier, less charged fly-ash particles will deviate little from the path determined by the vibratory feeder, whereas the lighter, more heavily charged carbon particles will tend also to be moved in a lateral direction under the influence of the alternating field. Bins or other receptacles are placed at appropriate points with respect to the first electrode for the collection of PFA-rich fractions and carbon-rich fractions.
Although the above-described apparatus represented a significant advance in the art, it has since been found that its operation can be improved in a number of respects.
It has been found that the width of the lower conveyor electrode is limited by the range of action of the oscillating electric field generated by the upper electrode. The intensity of the electric field is determined by the voltage applied to the upper electrode and, for any given region of the field, by the local distance between the upper and the lower electrodes. Owing to the angle between the two electrodes, the distance between the upper and lower electrode increases in the width-wise direction. As the electrodes diverge, there is a corresponding decrease in the electric field intensity. An attempt to increase the field intensity by increasing the potential applied to the upper electrode would significantly increase the likelihood of electrical breakdown (sparkover), in particular in the region of minimum distance between the upper and lower electrodes.